1. Field of the Invention
The present invention relates to a control of an automatic transmission in a vehicle, and more particularly, to a control of a friction engaging means such as a clutch and a brake in the speed stage shifting of an automatic transmission by an electronic computer through a hydraulic actuation system.
2. Description of the Prior Art
It is already known in the art of automatic transmission in a vehicle having an assembly of gear wheels such as a planetary gear mechanism and friction engaging means such as clutches and brakes adapted to be selectively engaged or disengaged so as to set up various speed stages providing various reduction gear ratios in the automatic transmission to control said engagement and disengagement of the friction engaging means by a combination of an electric control means and a hydraulic actuation system, wherein said electric control means dispatches electric control signals for selective setting up of certain speed stages in the transmission, said electric control signals being supplied to one or more solenoid valves incorporated in said hydraulic actuation system so as to change over supply or exhaust of a hydraulic pressure to or from said friction engaging means, as disclosed in Japanese Patent Laid-open Publication 57-86650.
Further, in such an art of electro-hydraulic control of the automatic transmission in a vehicle it is also known to control supply or exhaust of a hydraulic pressure to or from certain friction engaging means in changing over of speed stages of the transmission under watching of the rotational speed of a rotation member such as a drum of a clutch incorporated in the gear train so that the supply or exhaust of the hydraulic pressure to or from such certain friction engaging means is controlled on a real time basis according to each instant difference between the rotational speed of said rotation member and a certain target value therefor computed by an electronic computer based upon data with regard to various operation parameters of the vehicle in a manner of feedback control cyclically repeated at a certain time interval such as several microseconds, as disclosed in Japanese Patent Laid-open Publication 60-201152.
However, in such a combination of an electronic computer system and a hydraulic actuation system there is a great difference between the speeds of operation of the two systems. The electronic computer can finish almost all control calculations at an instant, whereas the hydraulic actuation system needs a substantial time to accomplish certain control operations due to a hydraulic inertia which generally causes a relatively large delay in response to an increase or decrease of the cross sectional area of a flow passage allowed for the hydraulic fluid. Therefore, if, for example, the electronic computer system generates a control output signal to change over the transmission from a certain first speed stage to a certain second speed stage at a certain moment to, as shown in FIG. 1, the hydraulic pressure in a clutch or a brake which is newly supplied with the hydraulic pressure for setting up said certain second gear stage is delayed as shown in FIG. 1, wherein the delay includes a time lag Tg which lapses before the hydraulic pressure starts to increase and an asymptotic delay Tf such as a first order delay, a second order delay or a further order delay during which the hydraulic pressure gradually increases so as asymptotically to approach the final value thereof.
In the electro-hydraulic control of the automatic transmission in a vehicle the electronic control system generally provides its output control signal in the form of a series of electric pulses to be supplied to a solenoid valve in the hydraulic actuation system, said pulse signal having a duty ratio representing the instructions to be given to the hydraulic actuation system. If the electronic computer system operates to produce an output signal such as said pulse signal having a variable duty ratio based upon only such input data as throttle opening of the engine determined by stepping on an accelerator pedal by the driver, patterns of driving such as the economy pattern putting preference on the economical performance of the vehicle and the power pattern putting preference on the power performance of the vehicle selected at a pattern select switch operated by the driver, and shift positions such as the R, N, D, 2 and L set up by a manual lever handled by the driver, the delay in response of the hydraulic actuation system as shown in FIG. 1 would cause no substantial problem. However, if the control computation in the electronic computer system also refers to such data as vehicle speed, oil temperature in the engine, rotational speed of a certain rotation member so as to modify the curve of asymptotic approach of the hydraulic pressure to its target value for a better speed stage shifting performance of the automatic transmission, the delay in the hydraulic actuation system seriously affects the performance of the automatic transmission control as the vehicle speed and the rotational speed of said certain rotation member are directly affected by such delay in the operation of the hydraulic actuation system thereby substantially disturbing the feedback gain. Therefore, certain measures are required to deal with such delays in the hydraulic actuation system if high performance of the automatic transmission is to be ensured based upon wide variety of operation parameters having more or less influence on the operation of the automatic transmission.